Anomalously high abundance of Crocosphaera in the South Pacific Gyre /Benavides, Mar Caffin, M. Duhamel, S. Foster, R. A. Grosso, O. Guieu, C. Van Wambeke, F. /Bonnet, Sophie Crocosphaera oligotrophic diazotrophs cyanobacteria We find diazotrophically active Crocosphaera in the clearest waters of the world's oceans. The unicellular diazotrophic cyanobacterium Crocosphaera contributes significantly to fixed nitrogen inputs in the oligotrophic ocean. In the western tropical South Pacific Ocean (WTSP), these diazotrophs abound thanks to the phosphorus-rich waters provided by the South Equatorial Current, and iron provided aeolian and subsurface volcanic activity. East of the WTSP, the South Pacific Gyre (SPG) harbors the most oligotrophic and transparent waters of the world's oceans, where only heterotrophic diazotrophs have been reported before. Here, in the SPG, we detected unexpected accumulation of Crocosphaera at 50 m with peak abundances of 5.26 x 10(5) nifH gene copies l(-1). The abundance of Crocosphaera at 50 m was in the same order of magnitude as those detected westwards in the WTSP and represented 100% of volumetric N-2 fixation rates. This accumulation at 50 m was likely due to a deeper penetration of UV light in the clear waters of the SPG being detrimental for Crocosphaera growth and N-2 fixation activity. Nutrient and trace metal addition experiments did not induce any significant changes in N-2 fixation or Crocosphaera abundance, indicating that this population was not limited by the resources tested and could develop in high numbers despite the oligotrophic conditions. Our findings indicate that the distribution of Crocosphaera can extend into subtropical gyres and further understanding of their controlling factors is needed. 2022 text https://www.documentation.ird.fr/hor/fdi:010084676 fdi:010084676 Benavides Mar, Caffin M., Duhamel S., Foster R. A., Grosso O., Guieu C., Van Wambeke F., Bonnet Sophie. Anomalously high abundance of Crocosphaera in the South Pacific Gyre. 2022, 369 (1), fnac039 [7 ] EN PACIFIQUE